I highly recommend reading Das Kapital at some point. It’s a fairly tough read, but definitely worth it to get a serious grounding in theory. Most serious leftist theory works relate to it one way or another making it fundamental material.

However, Wage Labour and Capital and Value, Price and Profit both encompass the central points of Das Kapital while being much more concise and easy to read.

I also highly recommend Lenin’s Imperialism, the Highest Stage of Capitalism and The State and Revolution. The first is a good intro to the mechanics of capitalism that lead to monopolies, capital concentration, colonization, and wars. The second discusses why reformism within the system is futile and the need for revolutionary change. Both are essential works in my opinion. Meanwhile, Lenin’s “Left-Wing” Communism: an Infantile Disorder reflects on how Bolsheviks built their movement and advice on power building for a serious working class party.

For more modern works I think that Michael Parenti’s Blackshirts and Reds and Inventing Reality are must reads.